Contact pads in many semiconductor applications require a top layer that is formed from one type of material and a bottom layer that is formed from another type of material. The top layer is used for electrically connecting the semiconductor device to an external element, whereas the bottom layer is used for electrically connecting and mechanically attaching the contact pads to the underlying epitaxial structure of the semiconductor device. Unfortunately, the different materials that are used to form the top and bottom layers of the contact pads can adversely impact one another. For example, if the top layer is formed from aluminum (Al) and the bottom layer is formed from titanium (Ti), the aluminum (Al) of the top layer readily diffuses into the titanium (Ti) of the bottom layer at high fabrication and operating temperatures. Such diffusion leads to significant reliability issues with regard to the contact pads and the semiconductor device as a whole. To combat diffusion, designers may provide a barrier layer between the top and bottom layers of the contact pads. The material composition of the barrier layer is selected to prevent, or at least significantly inhibit, the diffusion of the material of the top layer into the material of the bottom layer, and vice versa.
The various layers of the contact pads are formed sequentially using thin-film deposition processes, such as sputtering and evaporative deposition processes. Given the relative ease of combining numerous elements and the nature of the chemical reaction during deposition, sputtering processes often produce a more effective barrier layer. However, sputtered layers do not lend themselves to lift-off processing where patterned masks are employed prior to depositing the metal layers and removed after the metal layers have been deposited to form contact pads with desired shapes and at desired locations. In particular, when lift-off processing is used in conjunction with a sputtering process, removal of the patterned mask and the portions of the metal layers that reside above the patterned mask is difficult. Further, the contact pads are left with residual tags and other artifacts that are undesirable. In contrast to sputtering processes, evaporative processes do lend themselves to lift-off processing, but have historically not provided barrier layers as effective as sputter processes.
Accordingly, there is a need for an effective barrier layer for contact pads in semiconductor devices. There is a further need for an effective barrier layer that can be reliably formed when lift-off processing is employed.